Threaded Fasteners (Screwed Joints)

Threaded Fasteners (Screwed Joints)

Introduction:

This blog is intended to cover the descriptions, uses and materials of threaded fasteners used in practice. Typical methods of fastening or joints use such as devices as bolts, nuts, cap screws, setscrews, rivets, spring retainers, locking devices, pins, keys, welds, and adhesives.

One of the important targets of mechanical design engineers is to reduce the number of fasteners. For example, Jumbo jets such as Boeing’s 747 require as many as 2.5 million fasteners, some of which cost several dollars per piece. To keep cost down, aircraft manufacturers, and their subcontractors, constantly review new fastener design, installation techniques, and tooling.

The helical-thread screw was undoubtedly an extremely important mechanical invention. It is the basis of power screws and threaded fasteners, an important component in nonpermanent joints.

The terminology of screw threads is explained below:

·        The Pitch is the distance between adjacent thread forms measured parallel to the thread axis.

·        The Major diameter is the largest diameter of a screw thread.

·        The Minor or root diameter is the smallest diameter of the screw thread.

·        The Pitch diameter is the imaginary diameter between the major and minor diameters.

·        The Lead is the distance that nut advances parallel to the screw axis when the nut is given one turn. For a single start thread, the lead is the same as the pitch.

Screwed Joints:

A screw thread is formed by cutting a continuous helical groove on a cylindrical surface. Standard screw threads consist of the unified inch series and the metric series. There are two standard profiles in the metric series, both the Unified and merc M threads utlize the same profile, namely:

1.      M

2.      MJ

A screw made by cutting a single helical groove on the cylinder is known as single threaded (or single-start) screw and if a second thread is cut in the space between the grooves of the first, a double threaded (or double-start) screw is formed. Similarly, triple and quadruple (i.e. multiple-start) threads may be formed. The helical grooves may be cut either right hand or left hand commonly known as right handed or left handed screw.

It is mainly composed of two elements i.e. a nut and bolt. The screwed joints are widely used where the machine parts are required to be readily connected or disconnected without damage to the machine or the fastening. Following are the advantages and disadvantages of the screwed joints.

Advantages

1. Screwed joints are highly reliable in operation and easy to operate.

2. Screwed joints are convenient to assemble and disassemble.

3. A wide range of screwed joints may be adapted to various operating conditions.

4. Screws are relatively cheap to produce due to standardization and highly efficient manufacturing processes.

Disadvantages

The main disadvantage of the screwed joints is the stress concentration in the threaded portions which are vulnerable points under variable load conditions.

Stresses in Screwed Fastening due to Static Loading

The following stresses in screwed fastening are induced due to static loading are important from the subject point of view:

1. Internal stresses due to screwing up forces,

2. Stresses due to external forces, and

3. Stress due to combination of stresses.

Stresses due to External Forces

When bolt is subjected to an external load, the following stresses are induced in it:

1.      Tensile stress.

The bolts and screws usually carry a load in the direction of the bolt axis which produces a tensile stress in them.

2.      Shear stress.

Nut and bolts are used to prevent the relative movement of two or more parts, as in case of flange coupling, and then the shear stress is induced in the bolts. When the bolts are subjected to direct shearing loads, they should be located in such a way that the shearing load comes upon the body (i.e. shank) of the bolt and not upon the threaded portion. In some cases, the bolts may be relieved of shear load by using shear pins. When a number of bolts are used to share the shearing load, the finished bolts should be fitted to the reamed holes.

3.      Combined tension and shear stress.

When the bolt is subjected to both tensile and shearing loads, as in case of coupling bolts or bearing, then

·        The diameter of the shank of the bolt is obtained from the shear load and

·        That of threaded part from the tensile load.

A diameter slightly larger than that required for either shear or tension may be assumed and stresses due to combined load should be checked for the following principal stresses. When Factor of safety is taken into account, diameter should be increased.